Half a cube -- https://martin-ueding.de/posts/half-cube/

projection.scad

module mycube() {
    cube(size = 20, center = true);
};

module myplane(thickness) {
    color("black", alpha = 0.1)
        rotate([0, 0, 180])
        rotate([0, 0, 135])
        rotate([-145, 0, 0])
        translate([-20, 0, -20])
        cube([40, thickness, 40], center = false);
};

function gamma (beta) = 1 / sqrt(1 + beta*beta);

beta = 0.0;

multmatrix([[1, beta, beta, 0],
        [beta, 1, beta, 0],
        [beta, beta, 1, 0],
        [0, 0, 0, 1]])
{
    difference() {
        mycube();
        myplane(100);
    }

    myplane(1);
}